Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide a method and an apparatus for video coding. In some examples, an apparatus includes processing circuitry that obtains a plurality of control point motion vectors for a current block, determines first motion vectors and second motion vectors for a plurality of sub-blocks of the current block according to the plurality of control point motion vectors. The first motion vectors correspond to a first relative position in each sub-block. At least one first motion vector is different from a corresponding second motion vector. The processing circuitry obtains a first set of predicted samples according to the first motion vectors, obtains a second set of predicted samples according to the second motion vectors, and obtains a third set of predicted samples for the current block based on the first set of predicted samples and the second set of predicted samples.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 62/812,221, “STACKED AFFINE INTER PREDICTIONMETHODS” filed on Feb. 28, 2019, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described herein is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five neighboring blocks,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus includes processingcircuitry that obtains a plurality of control point motion vectors for acurrent block, the current block being divided into a plurality ofsub-blocks. The processing circuitry determines first motion vectors forthe plurality of sub-blocks, respectively, according to the plurality ofcontrol point motion vectors, the first motion vectors corresponding toa first relative position in each sub-block. The processing circuitryalso determines second motion vectors for the plurality of sub-blocks,respectively, according to the plurality of control point motionvectors, at least one first motion vector from the first motion vectorsbeing different from a corresponding second motion vector from thesecond motion vectors. The processing circuitry obtains a first set ofpredicted samples for the current block according to the first motionvectors and the plurality of sub-blocks, obtains a second set ofpredicted samples for the current block according to the second motionvectors and the plurality of sub-blocks, and obtains a third set ofpredicted samples for the current block based on the first set ofpredicted samples and the second set of predicted samples.

In some embodiments, the second motion vectors correspond to a secondrelative position in each sub-block, the first relative position beingdifferent from the second relative position. In one example, the firstrelative position is a center of each sub-block. In one example, thesecond relative position is a particular corner of each sub-block.

In some embodiments, the first relative position and the second relativeposition are symmetric with respect to one of a vertical line, ahorizontal line, and a diagonal line intersecting a center of eachsub-block. In some embodiments, the first relative position is a centerof a left edge of each sub-block, and the second relative position is acenter of a right edge of each sub-block.

In some embodiments, the second motion vectors are obtained by applyinga motion vector offset to the first motion vectors.

In some embodiments, the third set of predicted samples is calculated asa weighted average of the first set of predicted samples and the secondset of predicted samples. In one example, a first pixel in the first setof predicted samples for a particular sub-block is located at a firstposition in the sub-block and has a first weight for calculating thecombination, a second pixel in the first set of predicted samples forthe particular sub-block is located at a second position in thesub-block and has a second weight for calculating the combination, andthe first weight is greater than the second weight, and the firstposition is closer to the first relative position of the sub-block thanthe second position.

In one example, one of the plurality of sub-blocks has a size of 4×4pixels. In such example, a pixel in the first set of predicted samplesfor the one of the plurality of sub-blocks has a weight of three over atotal weight of four when the pixel is located less than three pixelsaway from the first relative position along a horizontal direction or avertical direction, and a weight of one over the total weight of fourwhen the pixel is located three or more pixels away from the firstrelative position along the horizontal direction or the verticaldirection.

In one example, weights for calculating the weighted average for aparticular sub-block are derived according to a generalizedbi-prediction (GBi) index for the particular sub-block.

In some embodiments, the current block is a uni-predicted block. In someembodiments, a de-blocking process is not performed on the currentblock.

In some embodiments, the processing circuitry also determines whether astacked affine mode is enabled in a coding region of a particular levelaccording to a flag signaled at the particular level, where the currentblock is included in the coding region of the particular level. Theparticular level can correspond to one of a slice, title, title-group,picture, and sequence level. The determining the second motion vectorsfor the plurality of sub-blocks and the obtaining the second set ofpredicted samples for the current block can be performed when thestacked affine mode is enabled. The determining the second motionvectors for the plurality of sub-blocks and the obtaining the second setof predicted samples for the current block can be not performed when thestacked affine mode is not enabled. In one example, when the flag thatis applicable to the coding region indicates that the stacked affinemode is enabled, the determining the second motion vectors for theplurality of sub-blocks and the obtaining the second set of predictedsamples for the current block are not performed on any bi-predictedblock in the coding region.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform any one or acombination of the methods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 is a schematic illustration of a current block and correspondingmerge candidates according to a merge mode in accordance with anembodiment.

FIG. 9 is a chart of pairs of merge candidates subject to a redundancycheck process in accordance with an embodiment.

FIG. 10 is a schematic illustration of motion vector scaling forderiving a temporal merge candidate in accordance with an embodiment.

FIG. 11 is a schematic illustration of predetermined points surroundingtwo starting points that are to be evaluated in a merge mode with motionvector difference (MMVD) in accordance with an embodiment.

FIG. 12A is a schematic illustration of a 6-parameter (according tothree control points) affine model in accordance with an embodiment.

FIG. 12B is a schematic illustration of a 4-parameter (according to twocontrol points) affine model in accordance with an embodiment.

FIG. 12C is a schematic illustration of motion vectors derived forsub-blocks of a current block coded according to an affine predictionmethod in accordance with an embodiment.

FIG. 13 is a schematic illustration of spatial neighboring blocks and atemporal neighboring block for a current block coded according to anaffine prediction method in accordance with an embodiment.

FIG. 14A is a schematic illustration of spatial neighboring blocks thatcan be used to determine predicted motion information for a currentblock using a subblock-based temporal motion vector prediction method inaccordance with one embodiment.

FIG. 14B is a schematic illustration of a selected spatial neighboringblock for a subblock-based temporal motion vector prediction method inaccordance with one embodiment.

FIG. 15A is a schematic illustration of two splitting examples for acurrent block according to a triangle prediction method in accordancewith one embodiment.

FIG. 15B shows an example of spatial and temporal neighboring blocksused to construct a uni-prediction candidate list for a triangleprediction method in accordance with an embodiment.

FIG. 16A shows an example of a coding unit applying a set of weights inan adaptive blending process in accordance with an embodiment.

FIG. 16B shows an example of a coding unit applying a set of weights inan adaptive blending process in accordance with an embodiment.

FIG. 17 is shows an interweaved affine prediction method in accordancewith an embodiment.

FIG. 18A shows an example of a set of weights for a sub-block in aninterweaved affine prediction method in accordance with an embodiment.

FIG. 18B is a schematic illustration of a pattern for dividing a blockinto sub-blocks in an interweaved affine prediction method in accordancewith an embodiment.

FIG. 19 is shows a stacked affine prediction method in accordance withan embodiment.

FIG. 20 is a schematic illustration of two sets of motion vectors for asame set of sub-blocks of a current block coded according to a stackedaffine prediction method in accordance with one embodiment.

FIG. 21 is a schematic illustration of coding a particular blockaccording to a stacked affine prediction method in accordance with anembodiment.

FIGS. 22A-22E shows various examples of assigning weights for asub-block in a stacked affine prediction method in accordance with oneor more embodiments.

FIG. 23 shows a flow chart outlining a process according to someembodiments of the disclosure.

FIG. 24 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Coding Encoder and Decoder

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures, and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230), and(240) may be illustrated as servers, personal computers, and smartphones, but the principles of the present disclosure may be not solimited. Embodiments of the present disclosure find application withlaptop computers, tablet computers, media players, and/or dedicatedvideo conferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230), and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks, and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3, can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example mega samples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any color space (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Inter Picture Prediction Modes

In various embodiments, a picture can be partitioned into blocks, forexample, using a tree structure based partition scheme. The resultingblocks can then be processed according to different processing modes,such as an intra prediction mode, an inter prediction mode (e.g., mergemode, skip mode, advanced motion vector prediction (AVMP) mode), and thelike. An intra coded block can be a block that is coded with an intraprediction mode. In contrast, an inter coded block can be a bock that isprocessed with an inter prediction mode.

1. Merge Mode

When a currently processed block, referred to as a current block, isprocessed according to the merge mode, a merge candidate list for thecurrent block can be constructed according to a set of spatially and/ortemporally neighboring blocks. One of the motion information candidatesin the merge candidate list can be selected for determining or derivingthe motion information for the current block. A merge index indicatingwhich candidate is selected can be signaled from an encoder to adecoder.

FIG. 8 is a schematic illustration of a current block and correspondingmerge candidates according to a merge mode in accordance with anembodiment. In this example, a merge candidate list is to be constructedfor a current block (810) that is to be processed according to the mergemode. A set of neighboring blocks, denoted A1, B1, B0, A0, B2, C0, andC1 are defined for the merge mode processing. Spatial candidates forconstructing the merge candidate list can be determined according tospatially neighboring blocks A1, B1, B0, A0, and B2 that are in the samepicture as the current block (810). Also, temporal candidates forconstructing the merge candidate list can be determined according totemporal neighboring blocks C0 and C1, which correspond to blocks thatare in another coded picture and neighbor or overlap a collocated blockof the current block (810). In one example, the temporal neighboringblock C1 can be located at a position corresponding to a position near(e.g., adjacent to) a center of the current block (810).

In some examples, the merge candidate list can have a predefined maximumnumber of merge candidates, represented as Cm. The merge candidates canbe listed in the merge candidate list according to a certain order. Inone example, according to a predefined order, a first number of mergecandidates, Ca, can be derived from the spatially neighboring blocksaccording to the order {A1, B1, B0, A0, B2}, and a second number ofmerge candidates, Cb=Cm−Ca, can be derived from the temporallyneighboring blocks according to the order {C0, C1}.

In some scenarios, candidate motion information from a particularneighboring block may be unavailable. For example, a neighboring blockcan be intra-predicted, outside of a slice or tile including the currentblock (810), or not in a same coding tree block (CTB) row as the currentblock (810). In some scenarios, candidate motion information fromvarious neighboring blocks may be redundant. In some examples, theredundant merge candidate can be removed from the merge candidate list(e.g., by performing a pruning process). When a total number ofavailable merge candidates (with redundant candidates being removed) inthe merge candidate list is smaller than the maximum number of mergecandidates Cm, one or more additional merge candidates can be added(e.g., according to a preconfigured rule) to fill the merge candidatelist. For example, additional merge candidates can include combinedbi-predictive candidates and/or zero motion vector candidates.

After the merge candidate list is constructed, at an encoder, anevaluation process can be performed to select a merge candidate from themerge candidate list. For example, rate-distortion (RD) performancecorresponding to each merge candidate can be calculated, and the onewith the best RD performance can be selected. Accordingly, a merge indexassociated with the selected merge candidate can be determined for thecurrent block (810) and signaled to a decoder.

At a decoder, the merge index of the current block (810) can bereceived. A similar merge candidate list construction process, asdescribed above, can be performed to generate a merge candidate listthat is the same as the merge candidate list generated at the encoderside. After the merge candidate list is constructed, a merge candidatecan be selected from the merge candidate list based on the receivedmerge index without performing any further evaluations in some examples.Motion information of the selected merge candidate can be used for asubsequent motion-compensated prediction of the current block (810).

A skip mode is also introduced in some examples. For example, in theskip mode, a current block can be predicted using a merge mode asdescribed above to determine a set of motion information, withoutintroducing residue information. A skip flag can be associated with thecurrent block. The skip flag and a merge index indicating the relatedmotion information of the current block can be signaled to a videodecoder. For example, at the beginning of a CU in an inter-pictureprediction slice, a skip flag can be signaled that implies thefollowing: the CU only contains one PU (2N×2N); the merge mode is usedto derive the motion information; and no residual information is presentin the bitstream. At the decoder side, based on the skip flag, aprediction block can be determined based on the merge index for decodinga respective current block without adding residue information. Thus,various methods for video coding with merge mode disclosed herein can beutilized in combination with a skip mode.

In some embodiments, a merge flag or a skip flag that is signaled in abitstream can indicate whether the current block (810) is to be codedaccording to the merge mode. If the merge flag is set to be TRUE, amerge index can then be signaled to indicate which candidate in a mergecandidate list will be used to provide motion information for thecurrent block. In some embodiments, up to four spatial merge candidates(from four spatially neighboring blocks) and up to one temporal mergecandidate (from one temporally neighboring block) can be added to themerge candidate list. A syntax MaxMergeCandsNum can be defined toindicate the size of the merge candidate list. The syntaxMaxMergeVandsNum can be signaled in the bitstream.

1.1 Redundancy Check for Spatial Candidates

In some embodiments, to reduce computational complexity for performing aredundancy check process, not all possible candidate pairs are subjectto the redundancy check process. Instead, in some examples, theredundancy check process can be limited to predetermine candidate pairs.A particular candidate can be added to the merge candidate list when theparticular candidate has motion information different from those ofother candidates in the corresponding candidate pairs.

FIG. 9 is a chart of pairs of merge candidates subject to the redundancycheck process in accordance with an embodiment. FIG. 9 shows thefollowing predetermined candidate pairs (denoted by their blockreferences) linked by arrows (902, 904, 906, 912, and 916), including{A1, B1} (linked by arrow 902), {A1, A0} (linked by arrow 904), {A1, B2}(linked by arrow 906), {B1, B0} (linked by arrow 912), and {B1, B2}(linked by arrow 916). In at least one embodiment, only the candidatepairs defined in FIG. 9 are subject to the redundancy check process.

1.2 Motion Vector Scaling for Temporal Candidate

FIG. 10 is a schematic illustration of motion vector scaling forderiving a temporal merge candidate in accordance with an embodiment. Inan embodiment, one temporal candidate can be added to the mergecandidate list. In some embodiments, in the derivation of the temporalmerge candidate for a current block (1012) in a current picture (1010),a scaled motion vector (1014) can be derived based on a collocated block(1022) included in a collocated reference picture (1020). A referencepicture list indicating which picture is used as the collocatedreference picture (1020) can be signaled in the bitstream, such as in aslice header in the bitstream. The scaled motion vector (1014) as thetemporal merge candidate for the current block (1022) can be scaled froma motion vector (1024) of the collocated block (1022) using pictureorder count (POC) distances, Tb and Td. Tb is defined to be a POCdifference between the current picture (1010) and a reference picture(1030) for the current picture (1010). Td is defined to be a POCdifference between the collocated picture (1020) and a reference picture(1040) for the collocated picture (1020). A reference picture index ofthe temporal merge candidate can be set to zero.

Moreover, the position for the temporal candidate can be selectedbetween temporal candidates from temporal neighboring blocks C0 and C1shown in FIG. 8. In some embodiments, a temporal candidate from thetemporal neighboring block C0 is first checked and used. If a temporalcandidate from the temporal neighboring block C0 is not available, suchas when the temporal neighboring block C0 is unavailable, intra coded,or is outside of a current row of CTUs, a temporal candidate from thetemporal neighboring block C1 can then be used.

1.3 History-Based Motion Vector Derivation

In some embodiments, one or more history-based MV prediction (HMVP)candidates can be added to the merge candidate list after the spatial MVcandidates and temporal MV candidate. The HMVP method includes storingthe motion information of one or more previously coded blocks in a tableand one or more HMVP candidates for the current block can be selectedfrom the table. The table with the one or more HMVP candidates can bemaintained during the encoding/decoding process. The table can be reset(emptied) when a new CTU row is to be processed. In some embodiments,whenever there is a non-subblock inter-coded CU, the associated motioninformation can be added to the last entry of the table as a new HMVPcandidate.

In some embodiments, a size of the HMVP table can be set to be 6, whichindicates up to 6 HMVP candidates can be included in the table. Wheninserting a new motion candidate to the table, a constrainedfirst-in-first-out (FIFO) rule can be utilized, with a redundancy checkis firstly applied to determine whether there is an identical HMVPalready in the table. If a particular one of the stored candidate in thetable is identical to a to-be-added HMVP candidate, the particular oneof the stored candidate can be removed from the table, and all otherHMVP candidates that come thereafter in the table can be moved up inresponse to the vacancy left by the removal of the particular one of thestored candidate.

In some embodiments, when the HMVP candidates are used in the mergecandidate list construction process, the latest one or more HMVPcandidates in the table can be checked according to a reversechronological order and inserted to the candidate list after thetemporal candidate. In some embodiments, a redundancy check can beperformed on the HMVP candidates against to the spatial or temporalmerge candidates already in the merge candidate list.

In some embodiments, to reduce the number of redundancy checkoperations, the redundancy check can be simplified by limiting a numberof HMVP candidates used in the merge candidate list constructionprocess. In some examples, the number of HMVP candidates used in themerge candidate list construction process can be set as (N<=4)?M: (8−N),wherein N indicates the number of existing candidates in the mergecandidate list, and M indicates the number of available HMVP candidatesin the table. In some embodiments, once the total number of candidatesin the merge candidate list reaches the maximally allowed mergecandidates minus 1, the process for adding candidates from HMVP to themerge candidate list can be terminated.

1.3 Pair-Wise Average Merge Candidates Derivation

In some embodiments, pairwise average candidates can be generated byaveraging predefined pairs of candidates in a current merge candidatelist. For example, the predefined pairs are defined as {(0, 1), (0, 2),(1, 2), (0, 3), (1, 3), (2, 3)} in an embodiment, where the numbersdenote the merge indices to the merge candidate list. In some examples,the averaged motion vectors are calculated separately for each referencepicture list. If both to-be-averaged motion vectors are available in onelist, these two motion vectors can be averaged even when they point todifferent reference pictures. If only one motion vector is available,the available one can be used directly. If no motion vector isavailable, the respective pair can be skipped in one example. Thepairwise average candidates replace the combined candidates in someembodiments in constructing a merge candidate list.

In some embodiments, when the merge candidate list is not full afterpair-wise average merge candidates are added, one or more zero MVs canbe added at the end of the merge candidate list until the maximum mergecandidate number is reached.

2. Merge with Motion Vector Difference (MMVD) Mode

In some embodiments, a merge with motion vector difference (MMVD) modeis used for determining a motion vector predictor of a current block.The MMVD mode can be used when skip mode or merge mode is enabled. TheMMVD mode reuses merge candidates on a merge candidate list of the skipmode or merge mode. For example, a merge candidate selected from themerge candidate list can be used to provide a starting point at areference picture. A motion vector of the current block can be expressedwith the starting point and a motion offset including a motion magnitudeand a motion direction with respect to the starting point. At an encoderside, selection of the merge candidate and determination of the motionoffset can be based on a search process (an evaluation process). At adecoder side, the selected merge candidate and the motion offset can bedetermined based on signaling from the encoder side.

In the MMVD mode, a selected merge candidate can be further refined bythe signaled motion vector difference (MVD) information. In someembodiments, the MVD information includes a merge candidate flag, anindex to specify motion magnitude, and an index for indication of motiondirection. In the MMVD mode, one of the candidates in the mergecandidate list can be selected to be used as a MV basis. The mergecandidate flag is signaled to specify which one is selected as the MVbasis.

The distance index can be used to specify a predefined offset for ahorizontal component or a vertical component of a starting point MV. Forexample, a plurality of predefined pixel distances are shown in Table 1each associated with indices from 0 to 7. The pixel distance having anindex of the distance index can be determined from the plurality ofpixel distances, and used to provide the motion magnitude. an offset isadded to either horizontal component or vertical component of startingMV. The relation of distance index and pre-defined offset is specifiedin Table 1.

TABLE 1 Distance Index Distance Index 0 1 2 3 4 5 6 7 Pixel ¼-pel ½-pel1-pel 2-pel 4-pel 8-pel 16-pel 32-pel Dis- tance

The direction index represents the direction of the MVD relative to thestarting point. For example, four directions with indices from 00 to 11(binary) are shown in Table 2. The direction with an index of thedirection index can be determined from the four directions, and used toprovide a direction of the motion offset with respect to the startingpoint.

TABLE 2 Direction Index Direction Index 00 01 10 11 x-axis + − N/A N/Ay-axis N/A N/A + −

In some embodiments, the definition of the MVD direction can varyaccording to the information of starting MVs. When the starting MVs isan un-prediction MV or bi-prediction MVs with both lists pointing to thesame side of the current picture (i.e. POCs of two reference picturesare both larger than the POC of the current picture, or are both smallerthan the POC of the current picture), the direction as identifiedaccording to Table 2 can be used to specify the direction of MV offsetadded to the starting MV. When the starting MVs is bi-prediction MVswith the two MVs pointing to the different sides of the current picture(i.e. the POC of one reference picture is larger than the POC of thecurrent picture, and the POC of the other reference picture is smallerthan the POC of the current picture), the direction as identifiedaccording to Table 2 can be used to add the MV offset for the list0 MVcomponent of the starting MV and the opposite direction for the list1 MVcomponent.

FIG. 11 is a schematic illustration of predetermined points surroundingtwo starting points that are to be evaluated in a merge mode with MMVDin accordance with an embodiment. In the example shown in FIG. 11, afirst and second motion vectors of a current block are associated withtwo reference pictures (1110) and (1140) in reference picture lists L0and L1, respectively. Two starting points (1112) and (1142) can bedetermined at the reference pictures (1110) and (1140).

In an example at the encoding side, based on the starting points (1112)and (1142), multiple predefined points extending from the startingpoints (1112) and (1142) in vertical directions (represented by +Y, or−Y) or horizontal directions (represented by +X and −X) in the referencepictures (1110) and (1140) can be evaluated. Based on the evaluations, amerge candidate can be selected from the merge list, and a motiondirection and a motion distance can also be determined. For example,based on the selected merge candidate, a base candidate index can bedetermined. Based on the selected motion vector predictor, such as thatcorresponding to the predefined point (e.g., 1116 or 1146), a directionand a distance of the point (e.g., 1116 or 1146) with respect to thestarting point (1112 or 1142) can be determined. According to Table 1and Table 2, a direction index and a distance index can accordingly bedetermined. In an example at the decoding side, based on the signaledstarting point, direction, and distance, a refined motion vectoraccording to MMVD can be determined.

3. Affine Prediction Mode

In some examples, a motion vector of a current block and/or sub-blocksof the current block can be derived using an affine model (e.g., a6-parameter affine model or a 4-parameter affine model). FIG. 12A is aschematic illustration of a 6-parameter (according to three controlpoints) affine model in accordance with an embodiment.

In an example, the 6 parameters of an affine coded block (e.g., currentblock 1202) can be represented by three motion vectors (also referred toas three control point motion vectors (CPMVs, e.g., CPMV₀, CPMV₁, andCPMV₂) at three different locations of the current block (e.g., controlpoints CP0, CP1, and CP2 at upper-left, upper-right, and lower-leftcorners in FIG. 12A). In some embodiments, for the 6-parameter affinemodel, a motion vector at a sample location (x, y) in the current block(1202) can be derived as:

$\begin{matrix}{\left\{ \begin{matrix}{{mv}_{x} = {{\frac{{mv_{1x}} - {mv_{0x}}}{W}x} + {\frac{{mv_{2x}} - {mv_{0x}}}{H}y} + {m\; v_{0x}}}} \\{{mv}_{y} = {{\frac{{mv_{1y}} - {mv_{0y}}}{W}x} + {\frac{{mv_{2y}} - {mv_{0y}}}{H}y} + {m\; v_{0y}}}}\end{matrix} \right.,} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where (mv_(0x), mv_(0y)) represents a motion vector of the upper-leftcorner control point (CPMV₀), (mv_(1x), mv_(1y)) represents a motionvector of the upper-right corner control point (CPMV₁), and (mv_(2x),mv_(2y)) represents a motion vector of the lower-left corner controlpoint (CPMV₂). Also, W represents a width of the current block (1202),and H represents a height of the current block (1202).

FIG. 12B is a schematic illustration of a 4-parameter (according to twocontrol points) affine model in accordance with an embodiment. Inanother example, a simplified affine model uses four parameters todescribe the motion information of an affine coded block (e.g., currentblock 1202), which can be represented by two motion vectors (alsoreferred to as two CPMVs, e.g., CPMV₀ and CPMV₁) at two differentlocations of the current block (e.g., control points CP0 and CP1 atupper-left and upper-right corners in FIG. 12B). In some embodiments,for the 4-parameter affine model, a motion vector at a sample location(x, y) in the current block (1202) can be derived as:

$\begin{matrix}{\left\{ \begin{matrix}{{mv}_{x} = {{\frac{{mv_{1x}} - {mv_{0x}}}{W}x} + {\frac{{mv_{1x}} - {mv_{0x}}}{H}y} + {m\; v_{0x}}}} \\{{mv}_{y} = {{\frac{{mv_{1y}} - {mv_{0y}}}{W}x} + {\frac{{mv_{1y}} - {mv_{0y}}}{H}y} + {m\; v_{0y}}}}\end{matrix} \right.,} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where (mv_(0x), mv_(0y)) represents a motion vector of the upper-leftcorner control point (CPMV₀), and (mv_(1x), mv_(1y)) represents a motionvector of the upper-right corner control point (CPMV₁). Also, Wrepresents a width of the current block (1202).

In some embodiments, in order to simplify the motion compensationprediction, subblock-based affine prediction method is applied. FIG. 12Cis a schematic illustration of motion vectors derived for sub-blocks ofa current block coded according to an affine prediction method inaccordance with an embodiment. In FIG. 12C, the current block (1202) canbe divided into sub-blocks. In this example, each sub-block can be a 4×4luma sub-block. Sub-block motion vectors (MVa-MVp) correspond to thecenters of the respective sub-blocks can be calculated according to4-parameter affine prediction method as described above, and rounded to1/16 fraction accuracy. The motion compensation predicted block data forthe sub-blocks can be generated according to the calculated sub-blockmotion vectors.

In some embodiments, the sub-block size of chroma-components can also beset to 4×4. The MV of a 4×4 chroma sub-block can be calculated as theaverage of the MVs of the four corresponding 4×4 luma sub-blocks.

In some embodiments, the CPMVs can be explicitly. In some embodiments,the CPMVs can be determined according to various CPMV predictionmethods, such as an affine merge mode or an affine AMVP mode.

3.1. Affine Merge Mode

FIG. 13 is a schematic illustration of spatial neighboring blocks and atemporal neighboring block for a current block (1301) coded according toan affine prediction method in accordance with an embodiment. As shown,the spatially neighboring blocks are denoted A0, A1, A2, B0, B1, B2, andB3 (1302, 1303, 1307, 1304, 1305, 1306, and 1308, respectively), and thetemporally neighboring block is denoted C0 (1312). In some examples, thespatially neighboring blocks A0, A1, A2, B0, B1, B2, and B3 and thecurrent block (1301) are in a same picture. In some examples, thetemporally neighboring block C0 is in a reference picture andcorresponds to a position outside the current block (1301) and adjacentto a lower-right corner of the current block (1301).

A list of motion information candidates (also referred to as an affinemerge candidate list) can be constructed using an affine merge modebased on motion information of one or more of the spatial neighboringblocks and/or temporal neighboring blocks. In some examples, the affinemerge mode can be applied when the current block (1301) has a width andheight that are equal to or greater than 8 samples. According to theaffine merge mode, the CPMVs of the current block (1301) can bedetermined based on the motion information candidates on the list. Insome examples, the list of motion information candidates can include upto five CPMV candidates, and an index can be signaled to indicate whichCPMV candidate is to be used for the current block. In some embodiments,a CPMV candidate includes all the CPMVs for an affine model.

In some embodiments, the affine merge candidate list can have threetypes of CPVM candidates, including inherited affine candidates,constructed affine candidates, and a zero MV. An inherited affinecandidate can be derived by extrapolation from the CPMVs of theneighboring blocks. A constructed affine candidate can be derived usingthe translational MVs of the neighboring blocks.

In an example, there can be at most two inherited affine candidates,which are derived from corresponding affine motion models of theneighboring blocks, including one block from left neighboring blocks (A0and A1) and one from upper neighboring blocks (B0, B1, and B2). For thecandidate from the left, neighboring blocks A0 and A1 can besequentially checked, and a first available inherited affine candidatefrom neighboring blocks A0 and A1 is used as the inherited affinecandidate from the left. For the candidate from the top, neighboringblocks B0, B1, and B2 can be sequentially checked, and a first availableinherited affine candidate from neighboring blocks B0, B1, and B2 isused as the inherited affine candidate from the top. In some examples,no pruning check is performed between the two inherited affinecandidates.

When a neighboring affine block is identified, a corresponding inheritedaffine candidate to be added to the affine merge list of the currentblock (1301) can be derived from the control point motion vectors of theneighboring affine block. In the FIG. 13 example, if the neighboringblock A1 is coded in affine mode, the motion vectors of the upper-leftcorner (control point CP0 _(A1)), the upper-right corner (control pointCP1 _(A1)), and the lower-left corner (control point CP2 _(A1)) of blockA1 can be obtained. When block A1 is coded using a 4-parameter affinemodel, the two CPMVs as an inherited affine candidate of the currentblock (1301) can be calculated according to the motion vectors ofcontrol point CP0 _(A1) and control point CP1 _(A1). When block A1 iscoded using a 6-parameter affine model, the three CPMVs as an inheritedaffine candidate of the current block (1301) can be calculated accordingto the motion vectors of control point CP0 _(A1), control point CP1_(m), and control point CP2 _(m).

Moreover, a constructed affine candidate can be derived by combining theneighboring translational motion information of each control point. Themotion information for the control points CP0, CP1, and CP2 is derivedfrom the specified spatial neighboring blocks A0, A1, A2, B0, B1, B2,and B3.

For example, CPMV_(k) (k=0, 1, 2, 3) represents the motion vector offour different control points, where CPMV₀ corresponds to control pointCP0, CPMV₁ corresponds to control point CP1, CPMV₂ corresponds tocontrol point CP2, and CPMV₃ corresponds to a temporal control pointbased on temporal neighboring block C0. For CPMV₀, neighboring blocksB2, B3, and A2 can be sequentially checked, and a first available motionvector from neighboring blocks B2, B3, and A2 is used as CPMV₀. ForCPMV₁, neighboring blocks B1 and B0 can be sequentially checked, and afirst available motion vector from neighboring blocks B1 and B0 is usedas CPMV₁. For CPMV₂, neighboring blocks A1 and A0 can be sequentiallychecked, and a first available motion vector from neighboring blocks A1and A0 is used as CPMV₂. Moreover, the motion vector of temporalneighboring block C0 can be used as CPMV₄, if available.

After CPMV₀, CPMV₁, CPMV₂, and CPMV₃, of four control points CP0, CP1,CP2 and the temporal control point are obtained, an affine mergecandidate list can be constructed to include affine merge candidatesthat are constructed in an order of: {CPMV₀, CPMV₁, CPMV₂}, {CPMV₀,CPMV₁, CPMV₃}, {CPMV₀, CPMV₂, CPMV₃}, {CPMV₁, CPMV₂, CPMV₃}, {CPMV₀,CPMV₁}, and {CPMV₀, CPMV₂}. Any combination of three CPMVs can form a6-parameter affine merge candidate, and any combination of two CPMVs canform a 4-parameter affine merge candidate. In some examples, in order toavoid a motion scaling process, if the reference indices of a group ofcontrol points are different, the corresponding combination of CPMVs canbe discarded.

In some embodiments, after inherited affine merge candidates andconstructed affine merge candidate are checked, if the list is still notfull, zero MVs are inserted to the end of the list.

3.2. Affine AMVP Mode

In some embodiments, a list of motion information candidates can beconstructed using an affine AMVP mode when the current block (1301) hasa width and height that are equal to or greater than 16 samples.According to the affine AMVP mode, an affine flag in the CU level can besignalled in the bitstream to indicate whether affine AMVP mode is used,and then another flag can be signaled to indicate whether a 4-parameteraffine or a 6-parameter affine model is used. In the affine AMVP mode,the difference of the CPMVs of the current block and corresponding CPMVpredictors (CPMVPs) can be signalled in the bitstream. In someembodiments, an affine AVMP candidate list can has a size of twocandidates is generated by using the following four types of CPVMcandidate in order, including (1) inherited affine AMVP candidates thatextrapolated from the CPMVs of one or more neighboring CUs; (2)constructed affine AMVP candidates that are derived using thetranslational MVs of one or more neighboring CUs; (3) translational MVsfrom one or more neighboring CUs; and (4) Zero MVs.

For deriving inherited affine AMVP candidates, in some examples, thechecking order of inherited affine AMVP candidates is the same as thechecking order of inherited affine merge candidates. The AVMP candidatescan be determined only from the affine CUs that have the same referencepicture as the current block. In some embodiments, no pruning process isapplied when inserting an inherited affine motion predictor into thecandidate list.

For deriving constructed affine AMVP candidates, in some examples, theconstructed AMVP candidate can be derived from one or more neighboringblocks as shown in FIG. 13 with the same checking order that is usedduring the affine merge candidate construction. In addition, thereference picture indices of the neighboring blocks are also checked.For example, the first block in the checking order that is inter codedand has the same reference picture as the current block can be used. Insome embodiments, there can be only one constructed affine AMVPcandidate. In some embodiments, when the current block is codedaccording to a 4-parameter affine mode, and the control point motionvectors of a checked neighboring block corresponding to CP0 and CP1thereof are both available, a collection of these control point motionvectors of the checked neighboring block can be added as a candidate inthe affine AMVP list. When the current block is coded according to a6-parameter affine mode, and the control point motion vectors of achecked neighboring block corresponding to CP0, CP1, and CP2 thereof areall available, a collection of these control point motion vectors of thechecked neighboring block can be added as a candidate in the affine AMVPlist. Otherwise, the constructed AMVP candidate from the checkedneighboring block can be determined as unavailable.

In some embodiments, if the affine AVMP candidate list has less than twocandidates after checking the inherited affine AMVP candidates andconstructed AMVP candidates, a candidate derived according totranslational MVs from one or more neighboring blocks can be added tothe affine AVMP candidate list. Finally, zero MVs can be used to fillthe affine AVMP candidate list if the list is still not full.

4. Subblock-Based Temporal Motion Vector Prediction (SbTMVP) Mode

FIG. 14A is a schematic illustration of spatial neighboring blocks thatcan be used to determine predicted motion information for a currentblock (1411) using a subblock-based temporal motion vector prediction(SbTMVP) method in accordance with one embodiment. FIG. 14A shows acurrent block (1411) and its spatial neighboring blocks denoted A0, A1,B0, and B1 (1412, 1413, 1414, and 1415, respectively). In some examples,spatial neighboring blocks A0, A1, B0, and B1 and the current block(1411) are in a same picture.

FIG. 14B is a schematic illustration for determining motion informationfor sub-blocks of the current block (1411) using the SbTMVP method basedon a selected spatial neighboring block, such as block A1 in thisnon-limiting example, in accordance with an embodiment. In this example,the current block (1411) is in a current picture (1410), and a referenceblock (1461) is in a reference picture (1460) and can be identifiedbased on a motion shift (or displacement) between the current block(1411) and the reference block (1461) indicated by a motion vector(1422).

In some embodiments, similar to a temporal motion vector prediction(TMVP) in HEVC, a SbTMVP uses the motion information in variousreference sub-blocks in a reference picture for a current block in acurrent picture. In some embodiments, the same reference picture used byTMVP can be used for SbTVMP. In some embodiments, TMVP predicts motioninformation at a CU level but SbTMVP predicts motion at a sub-CU level.In some embodiments, TMVP uses the temporal motion vectors from acollocated block in the reference picture, which has a correspondingposition adjacent to a lower-right corner or a center of a currentblock, and SbTMVP uses the temporal motion vectors from a referenceblock, which can be identified by performing a motion shift based on amotion vector from one of the spatial neighboring blocks of the currentblock.

For example, as shown in FIG. 14A, neighboring blocks A1, B1, B0, and A0can be sequentially checked in a SbTVMP process. As soon as a firstspatial neighboring block that has a motion vector that uses thereference picture (1460) as its reference picture is identified, such asblock A1 having the motion vector (1422) that points to a referenceblock AR1 in the reference picture (1460) for example, this motionvector (1422) can be used for performing the motion shift. If no suchmotion vector is available from the spatial neighboring blocks A1, B1,B0, and A0, the motion shift is set to (0, 0).

After determining the motion shift, the reference block (1461) can beidentified based on a position of the current block (1411) and thedetermined motion shift. In FIG. 10B, the reference block (1461) can befurther divided into 16 sub-blocks with reference motion information MRathrough MRp. In some examples, the reference motion information for eachsub-block in the reference block (1461) can be determined based on asmallest motion grid that covers a center sample of the respectivesub-block. The motion information can include motion vectors andcorresponding reference indices. The current block (1411) can be furtherdivided into 16 sub-blocks, and the motion information MVa through MVpfor the sub-blocks in the current block (1411) can be derived from thereference motion information MRa through MRp in a manner similar to theTMVP process, with temporal scaling in some examples.

The sub-block size used in the SbTMVP process can be fixed (or otherwisepredetermined) or signaled. In some examples, the sub-block size used inthe SbTMVP process can be 8×8 samples. In some examples, the SbTMVPprocess is only applicable to a block with a width and height equal toor greater than the fixed or signaled size, for example 8 pixels.

In an example, a combined sub-block based merge list which contains aSbTVMP candidate and affine merge candidates is used for the signalingof a sub-block based merge mode. The SbTVMP mode can be enabled ordisabled by a sequence parameter set (SPS) flag. In some examples, ifthe SbTMVP mode is enabled, the SbTMVP candidate is added as the firstentry of the list of sub-block based merge candidates, and followed bythe affine merge candidates. In some embodiments, the maximum allowedsize of the sub-block based merge list is set to five. However, othersizes may be utilized in other embodiments.

In some embodiments, the encoding logic of the additional SbTMVP mergecandidate is the same as for the other merge candidates. That is, foreach block in a P or B slice, an additional rate-distortion check can beperformed to determine whether to use the SbTMVP candidate.

5. Triangular Prediction

A triangular prediction mode (TPM) can be employed for inter predictionin some embodiments. In an embodiment, the TPM is applied to CUs thatare 8×8 samples or larger in size and are coded in skip or merge mode.In an embodiment, for a CU satisfying these conditions (8×8 samples orlarger sizes and coded in skip or merge mode), a CU-level flag issignaled to indicate whether the TPM is applied or not.

FIG. 15A is a schematic illustration of two splitting examples for acurrent block according to a triangle prediction unit mode in accordancewith one embodiment. When the TPM is used, in some embodiments, a CU issplit evenly into two triangle-shaped partitions, along a diagonal froman upper-left corner to a lower-right corner (e.g., a diagonal split) oralong a diagonal from an upper-right corner to a lower-left corner(e.g., an anti-diagonal split) as shown in FIG. 15A. In FIG. 15A, acurrent block (or also referred to as a CU) (1510A) can be split from anupper-left corner to a lower-right corner resulting in two triangularprediction units (1512) and (1514). The CU (1510B) can also be splitfrom an upper-right corner to a lower-left corner resulting in twotriangular prediction units (1516) and (1518). Each triangularprediction unit (1512), (1514), (1516), and (1518) in the two examples(1510A) and (1510B) can be inter-predicted using its own motioninformation.

In some embodiments, only uni-prediction is allowed for each triangularprediction unit. Accordingly, each triangular prediction unit has onemotion vector and one reference picture index. The uni-prediction motionconstraint can be applied to ensure that, similar to a conventionalbi-prediction method, not more than two motion compensated predictionsare performed for each CU. In this way, processing complexity can bereduced. The uni-prediction motion information for each triangularprediction unit can be derived from a uni-prediction merge candidatelist. In some other embodiments, bi-prediction is allowed for eachtriangular prediction unit. Accordingly, the bi-prediction motioninformation for each triangular prediction unit can be derived from abi-prediction merge candidate list.

In some embodiments, when a CU-level flag indicates that a current CU iscoded using the TPM, an index, referred to as triangle partition index,is further signaled. For example, the triangle partition index can havea value in a range of [0, 39]. Using this triangle partition index, thedirection of the triangle partition (diagonal or anti-diagonal), as wellas the motion information for each of the partitions (e.g., mergeindices (or referred to as TPM indices) to the respective uni-predictioncandidate list) can be obtained through a look-up table at the decoderside.

After predicting each of the triangular prediction unit based on theobtained motion information, in an embodiment, the sample values alongthe diagonal or anti-diagonal edge of the current CU are adjusted byperforming a blending process with adaptive weights. As a result of theblending process, a prediction signal for the whole CU can be obtained.Subsequently, a transform and quantization process can be applied to thewhole CU in a way similar to other prediction modes. Finally, a motionfield of a CU predicted using the triangle partition mode can becreated, for example, by storing motion information in a set of 4×4units partitioned from the CU. The motion field can be used, forexample, in a subsequent motion vector prediction process to construct amerge candidate list.

5.1 Uni-Prediction Candidate List Construction

In some embodiments, a merge candidate list for prediction of twotriangular prediction units of a coding block processed with a TPM canbe constructed based on a set of spatial and temporal neighboring blocksof the coding block. Such a merge candidate list can be referred to as aTPM candidate list with TPM candidates listed herein. In one embodiment,the merge candidate list is a uni-prediction candidate list.

FIG. 15B shows an example of spatial and temporal neighboring blocksused to construct a uni-prediction candidate list for a triangularprediction mode in accordance with an embodiment. In some embodiments,the uni-prediction candidate list includes five uni-prediction motionvector candidates in an embodiment. For example, the five uni-predictionmotion vector candidates are derived from seven neighboring blocksincluding five spatially neighboring blocks (denoted by S1 to S5 in FIG.15B) and two temporally neighboring blocks (denoted by T6 and T7 in FIG.15B).

In an example, the motion vectors of the seven neighboring blocks arecollected and put into the uni-prediction candidate list according tothe following order: first, the motion vectors of the uni-predictedneighboring blocks; (b) then, for the bi-predicted neighboring blocks,the L0 motion vectors (that is, the L0 motion vector part of thebi-prediction MV), the L1 motion vectors (that is, the L1 motion vectorpart of the bi-prediction MV), and averaged motion vectors of the L0 andL1 motion vectors of the bi-prediction MVs. In an example, if the numberof candidates is less than five, zero motion vectors are added to theend of the list. In some other embodiments, the merge candidate list mayinclude less than 5 or more than 5 uni-prediction or bi-prediction mergecandidates that are selected from candidate positions that are the sameor different from that shown in FIG. 15B.

In an embodiment, a CU is coded with a triangular partition mode with aTPM (or merge) candidate list including five TPM candidates.Accordingly, there are 40 possible ways to predict the CU when 5 mergecandidates are used for each triangular PU. In other words, there can be40 different combinations of split directions and merge (or TPM)indices: 2 (possible split directions)×(5 (possible merge indices for afirst triangular prediction unit)×5 (possible merge indices for a secondtriangular prediction unit)−5 (a number of possibilities when the pairof first and second prediction units shares a same merge index)). Forexample, when a same merge index is determined for the two triangularprediction units, the CU can be processed using a regular merge mode,instead of the triangular predication mode.

Accordingly, in an embodiment, a triangular partition index in the rangeof [0, 39] can be used to represent which one of the 40 combinations isused based on a lookup table as shown in Table 4 below.

TABLE 3 Look up table used to derive triangle direction and partitionmotions based on triangle index triangle_idx 0 1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 triangle dir 0 1 1 0 0 1 1 1 0 0 0 0 1 0 0 0 0 11 1 Part 1 cand 1 0 0 0 2 0 0 1 3 4 0 1 1 0 0 1 1 1 1 2 Part 2 cand 0 12 1 0 3 4 0 0 0 2 2 2 4 3 3 4 4 3 1 triangle_idx 20 21 22 23 24 25 26 2728 29 30 31 32 33 34 35 36 37 38 39 triangle dir 1 0 0 1 1 1 1 1 1 1 0 01 0 1 0 0 1 0 0 Part 1 cand 2 2 4 3 3 3 4 3 2 4 4 2 4 3 4 3 2 2 4 3 Part2 cand 0 1 3 0 2 4 0 1 3 1 1 3 2 2 3 1 4 4 2 4

The first row (triangle_idx) of Table 4 indicates the triangularpartition indices ranging from 0 to 39. The second row (triangle dir) ofTable 4 indicates possible split directions represented by 0 or 1. Thethird row (Part 1 cand) includes possible first merge indicescorresponding to a first triangular prediction unit and ranging from 0to 4. The fourth row (Part 2 cand) includes possible second mergeindices corresponding to a second triangular prediction unit and rangingfrom 0 to 4.

For example, when a triangular partition index having a value of 1 isreceived at a decoder, based on a column of the lookup tablecorresponding to triangle_idx=1, it can be determined that the splitdirection is a partition direction represented by the value of 1 (e.g.,the anti-diagonal split), and the first and second merge indices are 0and 1, respectively. As the triangle partition indices are associatedwith a lookup table, a triangle partition index is also referred to as atable index in this disclosure.

5.2 Adaptive Blending Along the Triangular Partition Edge

In an embodiment, after predicting each triangular prediction unit usingrespective motion information, a blending process is applied to the twoprediction signals of the two triangular prediction units to derivesamples around the diagonal or anti-diagonal edge. The blending processadaptively chooses between two groups of weighting factors depending onthe motion vector difference between the two triangular predictionunits. In an embodiment, the two weighting factor groups are as follows:

-   -   (1) First weighting factor group: {7/8, 6/8, 4/8, 2/8, 1/8} for        samples of a luma component and {7/8, 4/8, 1/8} for samples of        chroma component; and    -   (2) Second weighting factor group: {7/8, 6/8, 5/8, 4/8, 3/8,        2/8, 1/8} for samples of a luma component and {6/8, 4/8, 2/8}        for samples of a chroma component.

The second weighting factor group has more luma weighting factors andblends more luma samples along the partition edge.

In an embodiment, the following condition is used to select one of thetwo weighting factor groups. When reference pictures of the two trianglepartitions are different from each other, or when a motion vectordifference between the two triangle partitions is greater than athreshold (e.g., 16 luma samples), the second weighting factor group canbe selected. Otherwise, the first weighting factor group can beselected.

FIG. 16A shows an example of a coding unit applying the first set ofweighting factors in an adaptive blending process in accordance with anembodiment. In FIG. 16A, a first coding block (1610) includes lumasamples of a CU, and a second coding block (1620) includes chromasamples of the same CU. Each of the blocks is split into two triangularpartitions P1, P2, P1′, and P2′. A set of pixels along a diagonal edgein the coding block (1610) or (1620) are labeled with the numbers 1, 2,4, 6, and 7 corresponding to the weighting factors 1/8, 2/8, 4/8, 6/8,and 7/8, respectively, for partition P1 or partition P1′; and theweighting factors 7/8, 6/8, 4/8, 2/8, and 1/8, respectively, forpartition P2 or partition P2′. For example, for a pixel labelled withthe number of 2, a sample value of the pixel after a blending operationcan be obtained according to:

the blended sample value=2/8×S1+6/8×S2,  (Equation3)

where S1 and S2 represent sample values at the respective pixel butbelonging to predictions of a first triangular prediction unit P1 and asecond triangular prediction unit P2, respectively.

Moreover, the blank region without any weight represents that thepredicted samples according to the triangular partition P1 (or P1′) areadopted without merging. Similarly, the shaded region without any weightrepresents that the predicted samples according to the triangularpartition P2 are adopted without merging.

FIG. 16B shows an example of a coding unit applying a second set ofweighting factors in an adaptive blending process in accordance with anembodiment. In FIG. 16B, a third coding block (1630) includes lumasamples of a CU, and a fourth coding block (1640) includes chromasamples of the same CU. Each of the blocks is split into two triangularpartitions P1, P2, P1′, and P2′. A set of pixels along a diagonal edgein the coding block (1630) or (1640) are labeled with the numbers 1, 2,3, 4, 5, 6, and 7 corresponding to the weighting factors 1/8, 2/8, 3/8,4/8, 5/8, 6/8, and 7/8, respectively, for partition P1 or partition P1′;and the weighting factors 7/8, 6/8, 5/8, 4/8, 3/8, 2/8, and 1/8,respectively, for partition P2 or partition P2′. Also, the blank regionwithout any weight represents that the predicted samples according tothe triangular partition P1 (or P1′) are adopted without merging.Similarly, the shaded region without any weight represents that thepredicted samples according to the triangular partition P2 are adoptedwithout merging.

6. Combined Inter and Intra Prediction (CIIP)

In some embodiments, when a CU is coded in merge mode, and if the CUcontains at least 64 luma samples (e.g., a product of the CU width timesthe CU height is equal to or greater than 64), an additional flag can besignaled to indicate if a combined inter and intra prediction (CIIP)mode is applied to the current CU.

In some embodiments when the CIIP mode is applied, an intra predictionmode is first derived, where up to four possible intra prediction modescan be used, including a DC, planar, horizontal, or vertical mode.Afterwards, predicted samples for prediction blocks are derivedaccording to the inter prediction and intra prediction signals,respectively (i.e., a set of inter predicted samples and a set of intrapredicted samples), in a manner similar to regular intra and interprediction processes. Finally, the set of inter predicted samples andthe set of intra predicted samples can be combined according to aweighted averaging process to obtain a set of CIIP predicted samples.

6.1 Intra Prediction Mode Derivation

In an embodiment, up to 4 intra prediction modes, including the DC,Planar, Horizontal, and Vertical modes, can be used to predict the lumacomponent in the CIIP mode. In some embodiments, if the CU shape is verywide (that is, the width of the CU is more than two times of height ofthe CU), the horizontal mode may not be allowed. In some embodiments, ifthe CU shape is very narrow (that is, the height of the CU is more thantwo times of the width of the CU), the vertical mode may not be allowed.

In some embodiments, the CIIP mode can use three most probable modes(MPM) for intra prediction. The CIIP MPM candidate list can be formed asfollows:

-   -   (i) The left and top neighboring blocks are set as A and B,        respectively;    -   (ii) The intra prediction modes of block A and block B, denoted        as intraModeA and intraModeB, respectively, are derived as        follows:        -   a. Let X be either A or B,        -   b. intraModeX is set to DC if (1) block X is not available;            or (2) block X is not predicted using the CIIP mode or the            intra mode; or (3) block X is outside of the current CTU,            and        -   c. otherwise, intraModeX is set to (1) DC or Planar if the            intra prediction mode of block X is DC or Planar; or 2)            Vertical if the intra prediction mode of block X is a            “Vertical-like” angular mode (e.g., having a mode number            greater than 34 in some examples where 66 angular modes are            implemented), or (3) Horizontal if the intra prediction mode            of block X is a “Horizontal-like” angular mode (e.g., having            a mode number less than or equal to 34 in some examples            where 66 angular modes are implemented);    -   (iii) If intraModeA and intraModeB are the same:        -   a. If intraModeA is Planar or DC, then the three MPMs are            set to {Planar, DC, Vertical} in that order, and        -   b. Otherwise, the three MPMs are set to {intraModeA, Planar,            DC} in that order; and    -   (iv) Otherwise (intraModeA and intraModeB are different):        -   a. The first two MPMs are set to {intraModeA, intraModeB} in            that order, and        -   b. Uniqueness of Planar, DC, and Vertical is checked in that            order against the first two MPM candidate modes; as soon as            a unique mode is found, it is added as the third MPM.

In some embodiments, if the CU shape is very wide or very narrow asdefined above, the MPM flag can be inferred to be 1 without signaling.Otherwise, an MPM flag can be signaled to indicate if the CIIP intraprediction mode is being used.

In some embodiments, if the MPM flag is 1, an MPM index can be furthersignaled to indicate which one of the MPM candidate modes is used in theCIIP intra prediction. Otherwise, if the MPM flag is 0, the intraprediction mode can be set to a “missing mode” among the MPM candidatesdescribed above that is not included in the MPM candidate list. Forexample, since four possible intra prediction modes are considered inthe CIIP intra prediction mode, and the MPM candidate list contains onlythree intra prediction modes, one of the four possible modes can be themissing mode. For example, if the Planar mode is not in the MPMcandidate list, then Planar is the missing mode, and the intraprediction mode can be set to Planar.

For the chroma components, the DM mode can be applied without additionalsignaling. Therefore, in some examples, chroma components use the sameprediction mode as the corresponding luma components.

In some embodiments, the intra prediction mode of a CIIP-coded CU can bebe saved and used in the intra mode coding of the future neighboringCUs.

6.2 Combining Inter and Intra Predicted Samples

In an embodiment, the set of inter predicted samples in the CIIP modeP_(inter) is derived using the same inter prediction process applied toregular merge mode, and the set of intra predicted samples P_(intra) isderived using the CIIP intra prediction mode following the regular intraprediction process. Then, the intra and inter predicted samples can becombined using weighted averaging to obtain a set of CIIP predictedsamples P_(CIIP), where the weights can depend on the intra predictionmode and where the sample is located in the coding block.

In some embodiments, if the intra prediction mode is the DC or Planarmode, or if the block width or height is smaller than 4 pixels, equalweights can be applied to the set of intra predicted samples and the setof inter predicted samples. Otherwise, the weights can be determinedbased on the intra prediction mode (either Horizontal mode or Verticalmode in some cases) and the sample location in the block. For example,when the intra prediction mode is the Horizontal prediction mode, thecoding block can be split into four equal-area parts of a size of(W/4)×H, W being the width of the block and H being the height of theblock. Starting from the part closest to the intra prediction referencesamples and ending at the part farthest away from the intra predictionreference samples, the weight wt for the set of intra predicted sampleseach of the 4 regions can be set to 6, 5, 3, and 2, respectively, with atotal weight being 8. The weights for the Vertical mode are derivedsimilarly but the coding block is split into four equal-area parts of asize of W×(H/4).

With the derived weight wt for the set of intra predicted samples, inthis example, the combined CIIP prediction image can be derivedaccording to:

P _(CIIP)=((8−wt)×P _(inter) +wt×P _(intra)+4)>>3  (Equation 4)

III. Interweaved Affine Prediction

In some embodiments, interweaved affine prediction is used. FIG. 17shows an interweaved affine prediction method in accordance with anembodiment. As shown in FIG. 17, a current block (1710) with a size of16×16 samples is divided into sub-blocks with two different dividingpatterns, including first sub-blocks (1720) based on Pattern 0 andsecond sub-blocks (1730) based on Pattern 1. With Pattern 0, the currentblock (1710) is divided into first sub-blocks (1720) with an equal sizeof 4×4. In contrast, Pattern 1 is shifted by an offset (e.g., 2×2offset) with respect to Pattern 0. Two sets of auxiliary predictedsamples, including a set of predicted samples P0 (1740) and a set ofpredicted samples P1 (1750), corresponding to first sub-blocks (1720)and second sub-blocks (1730) are generated by affine motion compensation(AMC). For example, an affine model can be determined from an affinemerge candidate on a sub-block based merge candidate list. A sub-blockMV for each sub-block from the first sub-blocks (1720) and the secondsub-blocks (1730) can be derived based on the affine model. In someembodiments, the sub-block MVs can be determined according to centerpositions of the respective sub-blocks.

Thereafter, a set of combined predicted samples (1760) can be calculatedby combining the two sets of predicted samples P0 (1740) and P1 (1750).For example, a weighted average operation (1770) can be performed tocalculate a weighted average of two corresponding samples (denoted by P₀and P₁) in the two prediction images P0 (1740) and P1 (1750) pixel bypixel according to:

$\begin{matrix}\left\{ \begin{matrix}{{{{P = \left( {P_{0} + P_{1}} \right)}\operatorname{>>}1},{{{if}\mspace{20mu} w_{0}} = w_{1}},}\mspace{11mu}} \\{{{P = \left( {{w_{0}P_{0}} + {w_{1}P_{1}}} \right)}\operatorname{>>}2},{otherwise},}\end{matrix} \right. & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

where ω₀ and ω₁ are the weights corresponding to the pair of collocatedsamples in the two sets of predicted samples P0 (1740) and P1 (1750),respectively. A total weight (ω₀+ω₁) is four in one embodiment.

FIG. 18A shows an example of a set of weights for a sub-block in aninterweaved affine prediction method in accordance with an embodiment.In an embodiment, the weight of each sample in the weighted averageoperation (1770) can be set according to a pattern (1800) shown in FIG.18A. The pattern (1800) includes 16 samples included in a sub-block(e.g., a 4×4 sub-block in FIG. 17). Four prediction samples located nearthe center of the sub-block are associated with a weight value of 3,while 12 prediction samples located at the boundary of the sub-block areassociated with a weight value of 1. Depending on a position of a samplewithin a sub-block, a weight corresponding to the sample can bedetermined based on the pattern (1800).

FIG. 18B is a schematic illustration of a pattern for dividing a blockinto sub-blocks in an interweaved affine prediction method in accordancewith an embodiment. In an embodiment, to avoid block motion compensationon small sub-blocks, the interweaved prediction can only be applied onregions where the size of sub-blocks meets a threshold such as 4×4 forboth the two dividing patterns as shown in FIG. 17. For example, in theshaded area of the second sub-blocks (1730) as shown in FIG. 18B, nointerweaved prediction is applied, and in the non-shaded area of thesecond sub-blocks (1730), the interweaved prediction is applied.

In an embodiment, an interweaved prediction is applied on chromacomponents as well as the luma component. In some embodiments comparedwith other prediction methods, the interweaved prediction can beimplemented without increasing a memory access bandwidth, since an areaof a reference picture used for the AMC for all sub-blocks can befetched together as a whole, and therefore no additional readingoperation is needed.

Further, for flexibility, a flag can be signaled (e.g., in slice header)to indicate whether the interweaved prediction is used or not. In anexample, the flag is always signaled to be 1. In various embodiments,interweaved affine prediction can be applied on uni-predicted affineblocks, or on both uni-predicted and bi-predicted affine blocks.

IV. Stacked Affine Prediction

1. Overview

In some embodiments, the interweaved affine prediction method asdescribed above can be modified to become a stacked affine prediction.The stacked affine prediction can be used to achieve improvedcompression efficiency. FIG. 19 shows a stacked affine prediction methodin accordance with an embodiment. As shown in FIG. 19, a current block(1910) with a size (e.g., of 16×16 samples) can be divided intosub-blocks according to a dividing pattern for two auxiliarypredictions, including sub-blocks (1920A) for a first auxiliaryprediction and the same sub-blocks (1920B) for a second auxiliaryprediction, where the current block (1910) is divided into thesub-blocks with an equal size (e.g., of 4×4). In some embodiments, anaffine model for the current block (1910) can be determined as describedabove with reference to FIGS. 12A-12C, 13.

In the stacked affine prediction method, two different sets of sub-blockmotion vectors (e.g. Affine MV set 0 and Affine MV set 1) can be derivedusing the affine model for the sub-blocks (1920A) for the firstauxiliary prediction and the second sub-blocks (1920B) for the secondauxiliary prediction. In some embodiments, at least one sub-block motionvector for one of the sub-blocks is different from a second motionvector for the one of the sub-blocks.

FIG. 20 is a schematic illustration of two sets of motion vectors for asame set of sub-blocks of a current block coded according to a stackedaffine prediction method in accordance with one embodiment. For thesub-blocks (1920A) for the first auxiliary prediction, sub-block MVs(MVa1-MVp1) for each of the sub-blocks can be derived according to anaffine model defined by three control points (CP0, CP1, and CP2) andcorresponding control point motion vectors (CPMV₀, CPMV₁, and CPMV₂).For the sub-blocks (1920B) for the second auxiliary prediction,sub-block MVs (MVa2-MVp2) for each of the sub-block can be derivedaccording to the same affine model defined by the same three controlpoints (CP0, CP1, and CP2) and the same corresponding control pointmotion vectors (CPMV₀, CPMV₁, and CPMV₂). In some embodiments, thesub-block MVs (MVa1-MVp1) for the sub-blocks (1920A) (e.g., Affine MVset 0) can be determined as MVs corresponding to a first relativeposition in each sub-block. In some embodiments, the sub-block MVs(MVa2-MVp2) for the sub-blocks (1920B) (e.g., Affine MV set 1) can bedetermined as MVs corresponding to a second relative position in eachsub-block. In some embodiments, the sub-block MVs (MVa2-MVp2) for thesub-blocks (1920B) (e.g., Affine MV set 1) can be determined byprocessing the sub-block MVs (MVa1-MVp1).

Two sets of auxiliary predicted samples, including a set of predictedsamples P0 (1940) and a set of predicted samples P1 (1950),corresponding to the sub-blocks (1920A) for the first auxiliaryprediction and the sub-blocks (1920B) for the second auxiliaryprediction, respectively, are generated by affine motion compensation(AMC). For example, an affine model can be determined from an affinemerge candidate on a sub-block based merge candidate list. In someembodiments, a same reference index can be shared, and thus a samereference picture can be used, for generating the sets of auxiliarypredicted samples.

Thereafter, a set of combined predicted samples (1960) can be calculatedby combining the two sets of predicted samples P0 (1940) and P1 (1950).For example, a weighted average operation (1970) can be performed tocalculate a weighted average of two corresponding samples (denoted by P₀and P₁) in the two sets of predicted samples P0 (1940) and P1 (1950)pixel by pixel according to:

$\begin{matrix}\left\{ \begin{matrix}{{{{P = \left( {P_{0} + P_{1}} \right)}\operatorname{>>}1},{{{if}\mspace{20mu} w_{0}} = w_{1}},}\mspace{11mu}} \\{{{P = \left( {{w_{0}P_{0}} + {w_{1}P_{1}}} \right)}\operatorname{>>}2},{otherwise},}\end{matrix} \right. & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

where ω₀ and ω₁ are the weights corresponding to the pair of collocatedsamples in the two sets of predicted samples P0 (1740) and P1 (1750),respectively. A total weight (ω₀+ω₁) is four for example.

2. Embodiments for Sub-Block MV Derivation and Motion Compensation

In some embodiments, one affine MV set (e.g. Affine MV set 0) can bederived from the CPMVs according to the affine mode as MVs correspondingto a first relative position in each sub-block. In some embodiments, theother affine MV set (e.g. Affine MV set 1) can be derived from the CPMVsaccording to the affine mode as MVs corresponding to a second relativeposition in each sub-block.

In some embodiments, the first relative position (e.g., POS0) is acenter of each sub-block, and the second relative position (e.g., POS1)is a position other than the center of each sub-block. In one example,the second relative position (e.g., POS1) corresponds to an upper-leftsample of each sub-block. In one example, the second relative position(e.g., POS1) corresponds to an upper-right sample of each sub-block. Inone example, the second relative position (e.g., POS1) can be adaptivelyselected from the upper-left sample, the upper-right sample, or thelower-left sample of each sub-block, for example according to the CPMVvalues.

In one example, the second relative position (e.g., POS1) corresponds toan upper-left corner of each sub-block. In one example, the secondrelative position (e.g., POS1) corresponds to an upper-right corner ofeach sub-block. In one example, the second relative position (e.g.,POS1) can be adaptively selected from the upper-left corner, theupper-right corner, or the lower-left corner of each sub-block,according to the CPMV values.

In some embodiments, relative positions POS0 and POS1 can be inside thesub-block, on the boundary of the sub-block, or shared by neighboringsub-blocks. In some embodiments, relative positions POS0 and POS1 can besymmetric with respect to one of a vertical line, a horizontal line, anda diagonal line intersecting a center of each sub-block.

In one example for luma components, the relative position POS0 is (2, 0)with respect to the upper-left corner of each sub-block, and therelative position POS1 is (2, 3) with respect to the upper-left cornerof each sub-block.

In another example for luma components, the relative position POS0 is(0, 2) with respect to the upper-left corner of each sub-block, and therelative position POS1 is (3, 2) with respect to the upper-left cornerof each sub-block.

In another example for luma components, the relative position POS0 is(0, 0) with respect to the upper-left corner of each sub-block, and therelative position POS1 is (3, 3) with respect to the upper-left cornerof each sub-block.

In another example for luma components, the relative position POS0 is(0, 3) with respect to the upper-left corner of each sub-block, and therelative position POS1 is (3, 0) with respect to the upper-left cornerof each sub-block.

In another example for luma components, the relative position POS0 is(1, 1) with respect to the upper-left corner of each sub-block, and therelative position POS1 is (2, 2) with respect to the upper-left cornerof each sub-block.

In another example for luma components, the relative position POS0 andthe relative position POS1 can be adaptively determined, for exampleaccording to the values of the CPMVs.

In one particular example for luma components, the relative positionPOS0 is (0, 2) with respect to the upper-left corner of each sub-block,and the relative position POS1 is (4, 2) with respect to the upper-leftcorner of each sub-block. In this example, the sub-block MV at POS1 fora particular sub-block is indeed also used as the sub-block MV at POS0for a neighboring sub-block. In this example, the stacked affineprediction method may be simplified as further illustrated withreference to FIG. 21.

FIG. 21 is a schematic illustration of coding a particular blockaccording to a stacked affine prediction method in accordance with anembodiment. As shown in FIG. 21, a 16×16 affine block (2100) can bedivided into 16 sub-blocks of a size of 4×4 (B₀₀, B₀₁, B₀₂, B₀₃, B₁₀,B₁₁, B₁₂, B₁₃, B₂₀, B₂₁, B₂₂, B₂₃, B₃₀, B₃₁, B₃₂, and B₃₃). Sub-blockMVs (MV_(0,2), MV_(4,2), MV_(8,2), MV_(12,2), and MV_(16,2)) are derivedfor sub-blocks (B₀₀, B₁₀, B₂₀, and B₃₀) at position (0, 2), (4, 2), (8,2), (12, 2), and (16, 2) with respect to the upper-left corner of theblock (2100), respectively.

The prediction of a sub-block can be generated according to twosub-block MVs associated with the sub-block at (0, 2) and (4, 2) withrespect to the upper-left corner of each sub-block, which correspond toa center of a left edge and a center of a right edge of each sub-block.For example, MV_(0,2) and MV_(4,2) are used for sub-block B₀₀, andMV_(4,2) and MV_(8,2) are used for sub-block B₁₀. As the sub-block MVsmay be shared by neighboring sub-blocks, the generation of a 4×4prediction block of sub-block (B₀₀) according to MV_(4,2) and thegeneration of a 4×4 prediction block of sub-block (B₁₀) according toMV_(4,2) can be merged into generation of a 8×4 prediction blockaccording MV_(4,2). In some embodiments, since the per pixel predictioncomplexity of an 8×4 block is smaller than that of 4×4 block, theproposed method can reduce prediction complexity. For example, for abi-predicted affine block with N 4×4 sub-blocks in a row, the proposedmethod needs (N−1) 8×4 prediction blocks and two 4×4 prediction blocksfor the row. In contrast, N×2 4×4 prediction blocks are needed for thatrow if the process is implemented without the simplification asdescribed above.

Moreover, in some embodiments, one affine MV set (e.g. Affine MV set 0)can be derived from the CPMVs according to the affine mode as MVscorresponding to a relative (or predetermined relative) position in eachsub-block, such as a center of each sub-block. In some embodiments, theother affine MV set (e.g. Affine MV set 1) can be determined byprocessing the Affine MV set 0. For example, the Affine MV set 1 can beobtained by applying a motion vector offset ΔMV to the Affine MV set 0.

3. Embodiments for Weighted Average on Auxiliary Predicted Samples

In one embodiment, equal weights may be used for the averaging of thetwo sets of auxiliary predicted samples. In another embodiment, when asub-block MV is derived based on a position that is not at the center ofthe sub-block, a greater weight can be assigned to a pixel that iscloser to the corresponding relative position for deriving the sub-blockMV than a pixel that is farther away from the corresponding relativeposition.

FIGS. 22A-22E shows various examples of assigning weights for asub-block in a stacked affine prediction method in accordance with oneor more embodiments. In these examples, each sub-block has a size of 4×4pixels.

In some embodiments, a pixel in a set of predicted samples for asub-block has a weight of three over a total weight of four when thepixel is located less than three pixels away from the first relativeposition along a horizontal direction or a vertical direction, and aweight of one over the total weight of four when the pixel is locatedthree or more pixels away from the first relative position along thehorizontal direction or the vertical direction.

In one example as shown in FIG. 22A, when the sub-block MV is derivedaccording to the upper-left position (2210) of the sub-block, theupper-left four pixels of the sub-block can be assigned to a weight of3, and the other pixels can be assigned to a weight of 1, with a totalweight for the weighted averaging process is 4.

In one example as shown in FIG. 22B, when the sub-block MV is derivedaccording to the upper-right position (2220) of the sub-block, theupper-right four pixels of the sub-block can be assigned to a weight of3, and the other pixels can be assigned to a weight of 1, with a totalweight for the weighted averaging process is 4.

In one example as shown in FIG. 22C, when the sub-block MV is derivedaccording to the lower-left position (2230) of the sub-block, thelower-left four pixels of the sub-block can be assigned to a weight of3, and the other pixels can be assigned to a weight of 1, with a totalweight for the weighted averaging process is 4.

In one example as shown in FIG. 22D, when the sub-block MV is derivedaccording to the center of a left-edge position (2240) of the sub-block,the left eight pixels of the sub-block can be assigned to a weight of 3,and the other pixels can be assigned to a weight of 1, with a totalweight for the weighted averaging process is 4.

In one example as shown in FIG. 22E, when the sub-block MV is derivedaccording to the center of a right-edge position (2240) of thesub-block, the right eight pixels of the sub-block can be assigned to aweight of 3, and the other pixels can be assigned to a weight of 1, witha total weight for the weighted averaging process is 4.

In one embodiment, the stacked affine prediction may be applied onlywhen the current block is uni-predicted (e.g., only one referencepicture is used). In one embodiment, the stacked affine prediction maybe applied for both uni-predicted and bi-predicted blocks. In someembodiments, when the current block is bi-predicted, the stacked affineprediction can be applied to the motion compensation from a referencepicture of each prediction direction.

In some embodiments, when the stacked affine prediction is used, nodeblocking inside the block is performed on the current block. In atleast one embodiment, when the stacked affine prediction is used,deblocking inside the current block can still be performed.

In some embodiments, a generalized bi-prediction (GBi) index may be usedin conjunction with the stacked affine prediction, even when the stackedaffine prediction uses only one reference direction. In this example,the GBi index may be signaled to specify the weights for combining thetwo sets of auxiliary predicted samples. In one embodiment, when the GBiindex is used on a block coded according to the stacked affineprediction, the weighted average can only be performed according to theGBi index. In this case, other sub-block based weighted averaging maynot be performed.

4. Signaling of Stacked Affine Prediction

In some embodiments, a flag such as a stacked affine prediction flag(e.g., stacked affine flag) may be signaled. The flag can be signaled byhigh-level signaling (such as, but not limited to, at a slice, tile,tile-group, picture, or sequence level, etc.) to indicate whether thestacked affine prediction method is used.

In one embodiment, a flag such as sps_stacked_affine flag can besignaled at the SPS level. If this flag is true, a picture level or tilegroup level flag (e.g., picture_stacked_affine_flag) can be signaled toindicate whether the stacked affine prediction method is used for thecurrent decoded picture or tile group.

In one embodiment, the stacked affine prediction flag (e.g.,stacked_affine_flag) can be signaled at a level which is lower than thesequence level, such as at a picture level, tile group level, tilelevel, or block level, etc. In such case, the flag stacked_affine_flagmay be signaled only when an affine prediction enable flag signaled atthe sequence level is true. When the affine prediction enable flag issignaled as false at the sequence level, the flag stacked_affine_flagcan be inferred as false.

In one embodiment, the enable status of the stacked affine predictionmethod may be derived by other approaches, such as a predefined defaultsetting, etc., and may not necessarily be signaled.

In some embodiments, when the stacked affine prediction method isenabled and only applied to affine uni-prediction, the followingvariations may be implemented.

In one example, when the stacked affine prediction method is enabled fora coding region (such as a picture or a tile group), as indicated by acorresponding high level enable flag, only uni-predicted affine blocksare coded using the stacked affine prediction method, and bi-predictedaffine blocks are coded using regular affine bi-prediction. In such acase, the CU level syntax elements with respect to inter predictiondirection are not modified, and the semantic and/or the binarization ofinter prediction direction index is also not modified.

In one embodiment, when the stacked affine prediction method is enabledfor a coding region (such as a picture or a tile group), as indicated bya corresponding high level flag, only uni-prediction is allowed foraffine coded blocks. In this example, bi-prediction can be disabled foraffine coded blocks for the same coding region. In such case, thesemantic with respect to inter prediction direction, such asinter_pred_idc, may be modified to save bits.

FIG. 23 shows a flow chart outlining a process (2300) according to someembodiments of the disclosure. The process (2300) can be used inencoding or decoding a current block of a current picture, includingobtaining a set of predicted samples for a current block according to astacked affine prediction method. In some embodiments, one or moreoperations are performed before or after process (2300), and some of theoperations illustrated in FIG. 23 may be reordered or omitted.

In various embodiments, the process (2300) is executed by processingcircuitry, such as the processing circuitry in the terminal devices(210), (220), (230) and (240), the processing circuitry that performsfunctions of the video decoder (310), the processing circuitry thatperforms functions of the video decoder (410), and the like. In someembodiments, the process (2300) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (2300). The process startsat (S2301) and proceeds to (S2310).

At (S2310), a plurality of control point motion vectors for a currentblock is obtained. The current block is divided into a plurality ofsub-blocks. In some embodiments, the plurality of control point motionvectors (CPMVs) can be obtained according to an affine merge mode or anaffine ΔMVP mode as described above with reference to FIGS. 12A-12C, 13.In some embodiments, each sub-block can have a size of 4×4 pixels or 8×8pixels.

At (S2320), first motion vectors (e.g. Affine MV set 0) can bedetermined for the sub-blocks, respectively, according to the CPMVs.

At (S2330), second motion vectors (e.g. Affine MV set 1) can bedetermined for the sub-blocks, respectively, according to the CPMVs. Insome embodiments, at least a first motion vector for one of thesub-blocks being different from a second motion vector for the one ofthe sub-blocks.

In some embodiments, one affine MV set (e.g. Affine MV set 0) can bederived from the CPMVs according to the affine mode as MVs correspondingto a first relative position in each sub-block. In some embodiments, theother affine MV set (e.g. Affine MV set 1) can be derived from the CPMVsaccording to the affine mode as MVs corresponding to a second relativeposition in each sub-block. In some embodiments, the second relativeposition is a particular corner of each sub-block. In some embodiments,the first relative position and the second relative position aresymmetric with respect to one of a vertical line, a horizontal line, anda diagonal line intersecting a center of each sub-block.

In at least one example, the first relative position is a center of aleft edge of each sub-block, and the second relative position is acenter of a right edge of each sub-block.

Moreover, instead of deriving the two sets of sub-block motion vectorsaccording to two different relative positions POS1 and POS2, only onerelative position is used. For example, one affine MV set (e.g. AffineMV set 0) can be derived from the CPMVs according to the affine mode asMVs corresponding to a predetermined relative position in eachsub-block, such as a center of each sub-block. The other affine MV set(e.g. Affine MV set 1) can be determined by processing the Affine MV set0. For example, the Affine MV set 1 can be obtained by applying a motionvector offset ΔMV to the Affine MV set 0.

At (S2340), a first set of predicted samples for the current block isobtained according to the first motion vectors and the correspondingsub-blocks. In some embodiments, the first set of predicted samples canbe obtained by obtaining sub-block predicted data for each sub-blockaccording to the first motion vectors, respectively.

At (S2350), a second set of predicted samples for the current block isobtained according to the second motion vectors and the correspondingsub-blocks. In some embodiments, the second set of predicted samples canbe obtained by obtaining predicted data for each sub-block according tothe second motion vectors, respectively.

At (S2360), a third set of predicted samples for the current block canbe obtained according to a combination of the first set of predictedsamples and the second set of predicted samples. In some embodiments,the combination of the first set of predicted samples and the second setof predicted samples is calculated as a weighted average of the firstset of predicted samples and the second set of predicted samples.

In one example, when a first pixel in the first set of predicted samplesfor a particular sub-block is located at a first position in thesub-block and has a first weight for calculating the combination, asecond pixel in the first set of predicted samples for the particularsub-block is located at a second position in the sub-block and has asecond weight for calculating the combination, and the first position iscloser to the first relative position of the sub-block than the secondposition, the first weight can be greater than the second weight.

In one example, one of the sub-blocks has a size of 4×4 pixels, and apixel in the first set of predicted samples for one of the sub-blockshas (1) a weight of three over a total weight of four when the pixel islocated less than three pixels away from the first relative positionalong a horizontal direction or a vertical direction, and (2) a weightof one over the total weight of four when the pixel is located three ormore pixels away from the first relative position along the horizontaldirection or the vertical direction.

In some embodiments, weights for calculating the weighted average for aparticular sub-block are derived according to a generalizedbi-prediction (GBi) index for the particular sub-block.

In some embodiments, the current block is a uni-predicted block. In someembodiments, a de-blocking process is not performed on the currentblock.

In some embodiments, whether a stacked affine mode is enabled in acoding region of a particular level can be determined according to aflag signaled at the particular level, where the current block isincluded in the coding region of the particular level.

In some examples, the particular level corresponds to one of a slice,title, title-group, picture, and sequence level. In some examples, thedetermining the second motion vectors for the sub-blocks (S2330) and theobtaining the second set of predicted samples for the current block(S2350) are performed when the stacked affine mode is enabled. In oneexample, the determining the second motion vectors for the sub-blocks(S2330) and the obtaining the second set of predicted samples for thecurrent block (S2350) are not performed when the stacked affine mode isnot enabled.

In some examples, when the flag that is applicable to the coding regionindicates that the stacked affine mode is enabled, the determining thesecond motion vectors for the sub-blocks (S2330) and the obtaining thesecond set of predicted samples for the current block (S2350) are notperformed on any bi-predicted block in the coding region.

After (S2360), the process (2300) may proceed to (S2399) and terminate.

The embodiments described herein may be used separately or combined inany order. Further, each of the embodiments, encoder, and decoder may beimplemented by processing circuitry (e.g., one or more processors or oneor more integrated circuits). In one example, the one or more processorsexecute a program that is stored in a non-transitory computer-readablemedium.

V. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 24 shows a computersystem (2400) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 24 for computer system (2400) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (2400).

Computer system (2400) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (2401), mouse (2402), trackpad (2403), touchscreen (2410), data-glove (not shown), joystick (2405), microphone(2406), scanner (2407), camera (2408).

Computer system (2400) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (2410), data-glove (not shown), or joystick (2405), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (2409), headphones(not depicted)), visual output devices (such as screens (2410) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (2400) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(2420) with CD/DVD or the like media (2421), thumb-drive (2422),removable hard drive or solid state drive (2423), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (2400) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (2449) (such as, for example USB ports of thecomputer system (2400)); others are commonly integrated into the core ofthe computer system (2400) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (2400) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (2440) of thecomputer system (2400).

The core (2440) can include one or more Central Processing Units (CPU)(2441), Graphics Processing Units (GPU) (2442), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(2443), hardware accelerators for certain tasks (2444), and so forth.These devices, along with Read-only memory (ROM) (2445), Random-accessmemory (2446), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (2447), may be connectedthrough a system bus (2448). In some computer systems, the system bus(2448) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (2448),or through a peripheral bus (2449). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (2441), GPUs (2442), FPGAs (2443), and accelerators (2444) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(2445) or RAM (2446). Transitional data can be also be stored in RAM(2446), whereas permanent data can be stored for example, in theinternal mass storage (2447). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (2441), GPU (2442), massstorage (2447), ROM (2445), RAM (2446), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (2400), and specifically the core (2440) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (2440) that are of non-transitorynature, such as core-internal mass storage (2447) or ROM (2445). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (2440). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(2440) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (2446) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (2444)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   ΔMVP: Advanced Motion Vector Prediction-   ASIC: Application-Specific Integrated Circuit-   BMS: Benchmark Set-   CANBus: Controller Area Network Bus-   CD: Compact Disc-   CPMV: Control Point Motion Vector-   CPUs: Central Processing Units-   CRT: Cathode Ray Tube-   CTUs: Coding Tree Units-   CU: Coding Unit-   DVD: Digital Video Disc-   FPGA: Field Programmable Gate Areas-   GBi: Generalized Bi-prediction-   GOPs: Groups of Pictures-   GPUs: Graphics Processing Units-   GSM: Global System for Mobile communications-   HEVC: High Efficiency Video Coding-   HMVP: History-based Motion Vector Prediction-   HRD: Hypothetical Reference Decoder-   IC: Integrated Circuit-   JEM: Joint Exploration Model-   LAN: Local Area Network-   LCD: Liquid-Crystal Display-   LTE: Long-Term Evolution-   MMVD: Merge with MVD-   MV: Motion Vector-   MVD: Motion Vector Difference-   MVP: Motion Vector Predictor-   OLED: Organic Light-Emitting Diode-   PBs: Prediction Blocks-   PCI: Peripheral Component Interconnect-   PLD: Programmable Logic Device-   PUs: Prediction Units-   RAM: Random Access Memory-   ROM: Read-Only Memory-   SEI: Supplementary Enhancement Information-   SNR: Signal Noise Ratio-   SSD: Solid-State Drive-   SPS: Sequence Parameter Set-   SbTMVP: Subblock-based Temporal Motion Vector Prediction-   TMVP: Temporal Motion Vector Prediction-   TUs: Transform Units,-   USB: Universal Serial Bus-   VTM: Versatile Test Model-   VUI: Video Usability Information-   VVC: Versatile Video Coding

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof

What is claimed is:
 1. A method of video decoding in a decoder,comprising: obtaining a plurality of control point motion vectors for acurrent block, the current block being divided into a plurality ofsub-blocks; determining first motion vectors for the plurality ofsub-blocks, respectively, according to the plurality of control pointmotion vectors, the first motion vectors corresponding to a firstrelative position in each sub-block; determining second motion vectorsfor the plurality of sub-blocks, respectively, according to theplurality of control point motion vectors, at least one first motionvector from the first motion vectors being different from acorresponding second motion vector from the second motion vectors;obtaining a first set of predicted samples for the current blockaccording to the first motion vectors and the plurality of sub-blocks;obtaining a second set of predicted samples for the current blockaccording to the second motion vectors and the plurality of sub-blocks;and obtaining a third set of predicted samples for the current blockbased on the first set of predicted samples and the second set ofpredicted samples.
 2. The method of claim 1, wherein the second motionvectors correspond to a second relative position in each sub-block, thefirst relative position being different from the second relativeposition.
 3. The method of claim 2, wherein the first relative positionis a center of each sub-block.
 4. The method of claim 3, wherein thesecond relative position is a particular corner of each sub-block. 5.The method of claim 2, wherein the first relative position and thesecond relative position are symmetric with respect to one of a verticalline, a horizontal line, and a diagonal line intersecting a center ofeach sub-block.
 6. The method of claim 2, wherein the first relativeposition is a center of a left edge of each sub-block, and the secondrelative position is a center of a right edge of each sub-block.
 7. Themethod of claim 1, wherein the second motion vectors are obtained byapplying a motion vector offset to the first motion vectors.
 8. Themethod of claim 1, wherein the third set of predicted samples iscalculated as a weighted average of the first set of predicted samplesand the second set of predicted samples.
 9. The method of claim 8,wherein a first pixel in the first set of predicted samples for aparticular one of the sub-blocks is located at a first position in thesub-block and has a first weight for calculating the combination, asecond pixel in the first set of predicted samples for the particularone of the sub-blocks is located at a second position in the sub-blockand has a second weight for calculating the combination, and the firstweigh is greater than the second weight, and the first position iscloser to the first relative position of the sub-block than the secondposition.
 10. The method of claim 8, wherein one of the plurality ofsub-blocks has a size of 4×4 pixels, and a pixel in the first set ofpredicted samples for the one of the plurality of sub-blocks has aweight of three over a total weight of four when the pixel is locatedless than three pixels away from the first relative position along ahorizontal direction or a vertical direction, and a weight of one overthe total weight of four when the pixel is located three or more pixelsaway from the first relative position along the horizontal direction orthe vertical direction.
 11. The method of claim 8, wherein weights forcalculating the weighted average for a particular one of the pluralityof sub-blocks are derived according to a generalized bi-prediction (GBi)index for the particular one of the sub-blocks.
 12. The method of claim1, wherein the current block is a uni-predicted block.
 13. The method ofclaim 1, wherein a de-blocking process is not performed on the currentblock.
 14. The method of claim 1, further comprising: determiningwhether a stacked affine mode is enabled in a coding region of aparticular level according to a flag signaled at the particular level,the current block being included in the coding region of the particularlevel, wherein the particular level corresponds to one of a slice,title, title-group, picture, and sequence level, the determining thesecond motion vectors for the plurality of sub-blocks and the obtainingthe second set of predicted samples for the current block are performedwhen the stacked affine mode is enabled, and the determining the secondmotion vectors for the plurality of sub-blocks and the obtaining thesecond set of predicted samples for the current block are not performedwhen the stacked affine mode is not enabled.
 15. The method of claim 14,wherein when the flag that is applicable to the coding region indicatesthat the stacked affine mode is enabled, the determining the secondmotion vectors for the plurality of sub-blocks and the obtaining thesecond set of predicted samples for the current block are not performedon any bi-predicted block in the coding region.
 16. An apparatus,comprising: processing circuitry configured to: obtain a plurality ofcontrol point motion vectors for a current block, the current blockbeing divided into a plurality of sub-blocks; determine first motionvectors for the plurality of sub-blocks, respectively, according to theplurality of control point motion vectors, the first motion vectorscorresponding to a first relative position in each sub-block; determinesecond motion vectors for the plurality of sub-blocks, respectively,according to the plurality of control point motion vectors, at least onefirst motion vector from the first motion vectors being different from acorresponding second motion vector from the second motion vectors;obtain a first set of predicted samples for the current block accordingto the first motion vectors and the plurality of sub-blocks; obtain asecond set of predicted samples for the current block according to thesecond motion vectors and the plurality of sub-blocks; and obtain athird set of predicted samples for the current block based on the firstset of predicted samples and the second set of predicted samples. 17.The apparatus of claim 16, wherein the second motion vectors correspondto a second relative position in each sub-block, the first relativeposition being different from the second relative position.
 18. Theapparatus of claim 16, wherein the second motion vectors are obtained byapplying a motion vector offset to the first motion vectors.
 19. Theapparatus of claim 16, wherein the third set of predicted samples iscalculated as a weighted average of the first set of predicted samplesand the second set of predicted samples.
 20. A non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform: obtaining aplurality of control point motion vectors for a current block, thecurrent block being divided into a plurality of sub-blocks; determiningfirst motion vectors for the plurality of sub-blocks, respectively,according to the plurality of control point motion vectors, the firstmotion vectors corresponding to a first relative position in eachsub-block; determining second motion vectors for the plurality ofsub-blocks, respectively, according to the plurality of control pointmotion vectors, at least one first motion vector from the first motionvectors being different from a corresponding second motion vector fromthe second motion vectors; obtaining a first set of predicted samplesfor the current block according to the first motion vectors and theplurality of sub-blocks; obtaining a second set of predicted samples forthe current block according to the second motion vectors and theplurality of sub-blocks; and obtaining a third set of predicted samplesfor the current block based on the first set of predicted samples andthe second set of predicted samples.